Alloys with 4.5 to 5% magnesium together with additions of manganese and chromium attain, in the soft annealed condition, a tensile strength of 25 to 30 kg/mm.sup.2.
However, alloys with high magnesium content display certain peculiarities which must be taken into account during the manufacture of motor vehicle bodies by further cold working by combined deep drawing and stretch drawing.
Flow patterns (corresponding to a marked plastic zone after passing beyond the yield point) cannot be tolerated in external components of bodies. In addition after cold deformation and subsequent storage at elevated temperature, the sheet must not be sensitive to stress corrosion cracking (SCC). Likewise undesirable is the partial disappearance of the strength condition produced during the cold deformation because it is accompanied by a loss in strength and rigidity. Surprisingly, stress corrosion cracking has not been taken into account by the prior art in connection with the use of high magnesium alloy in the manufacture of vehicle bodies even though all the conditions are present to produce stress corrosion cracking namely the presence of deformation zones and internal stresses from deep drawing or stretch drawing, the normal subjection of the bodies to high temperatures (waste heat of the engine, incident sunlight), and the corrosive environment.
Treatments are known in the prior art to eliminate flow patterns. However, these treatments are such that they are not suitable for use on alloys to be used in motor vehicle bodies. These treatments produce a grain diameter above 50 .mu.m which, after cold deformation, leads to a so called orange peel effect on the surface of the cold-formed part, i.e., cold-deformation over the distinct flow zone of beyond about 1% remaining extension, which leads to a great loss in formability. Finally, quenching from a soft annealing temperature in the solution range of about 530.degree. C., brings about a further disadvantage because of the only transient effect which makes storage of the sheet practically impossible and therefore the sheet must be immediately deformed.
Likewise, measures are known for reducing the sensitivity of the high magnesium alloy to stress corrosion cracking. However, the combination of the measures for reducing SCC with the above mentioned treatments for avoiding flow patterns does not result in obtaining good resistance to SCC upon subsequent deformation.
Accordingly, it is the principle object of the present invention to provide an improved process for fabricating good formability, fine grain size aluminum sheet characterized by improved resistance to stress corrosion cracking.
It is a particular object of the present invention to fabricate aluminum sheet stock which is suitable for use in the manufacture of motor vehicle bodies by deep and stretch drawing.
Further objects and advantages will appear hereinbelow.